Numerous push-button control devices matching the above definition are known. By way of example, FIG. 1 illustrates a prior art device of this type. Referring to FIG. 1, middle part 1 of a watch has a recess 2 whose bottom has a bore 3 in which the push-button tube 4, which is fixed in the middle part, is engaged. A push-button stem 8 associated with a button 9 is arranged inside the tube so as to slide axially therein between a rest position and an active position. Button 9 includes a cap 5 that projects outside recess 2. It can be seen that cap 5 has an annular hollow 11 formed around a central cylindrical portion into which the threaded end of stem 8 is screwed. The stem and button 9 are returned to the rest position by a helical spring 13 surrounding the central cylindrical portion of cap 5. Spring 13 abuts, via one end thereof, against the bottom of annular hollow 11, whereas the other end thereof is engaged in tube 4. In the example illustrated, tube 4 is sealed in a water resistant manner by an O-ring gasket 6, which is arranged in annular hollow 11. The size of gasket 6 is such that it is compressed between the outer edge of the hollow and push-button tube 4. Button 9 also includes a ring 7, which closes the periphery of hollow 11 and which is provided for holding sealing gasket 6 in position.
One problem with devices of the type that has just been described is that it is generally impossible to adjust the dimension of recess 2 exactly to that of button 9. Indeed, with usual manufacturing tolerances, a gap has to be provided to act as a clearance space between button 9 and the wall of recess 2. One drawback of the presence of this gap is that it allows dirt to accumulate between pushbutton 9 and middle part 1. Moreover, even a slight error in centring button 9 relative to recess 2 can be detrimental to the attractiveness of the watch.
The above problem arises even more acutely in the frequent case where the middle part of the watch for which the push-button control device is intended, includes a cap used for covering a subjacent structure made of material that is easier to machine or cheaper. Such watchcases formed of a centre made of a first material covered with a protective or decorative cap made of a second material are well known to those skilled in the art. By way of example, EP Patent No. 1,102,136 discloses a watchcase including a centre made of steel covered with a cap made of very hard, scratchproof material, such as a ceramic material.
FIG. 2 illustrates, by way of example, a chronograph watch fitted with push-button control devices 29 and whose case is covered with a cap made of ceramic material, for example. With a watch of this type, the push-button stem naturally has to pass both through the cap and the centre of the watchcase. The cap and the centre of the case must therefore both have an aperture, and the aperture in the cap must be superposed, as precisely as possible, on the aperture in the centre of the case once the two components of the case are assembled. It is known, however, that the more difficult it is to machine the material in which the part is made, the greater the manufacturing tolerances of the machined part will normally be. Adjustment precision of the cap on the centre largely depends upon the precision with which the cap has been made.
FIG. 3 illustrates, by way of example, an alignment error between the aperture arranged in the steel centre and the aperture in cap 22. It is easy to see that insertion of a push-button control device into such poorly aligned apertures might cause problems. The pushbutton is even liable to be so off-centre that it simply does not pass through the aperture made in the ceramic cap. Moreover, even if the alignment error is not sufficient to prevent the pushbutton passing through, it may compromise the aesthetic qualities of the watch.